1) Intel may have the efficiency crown, but AMD has shown that they can be competitive enough that the gap can be overlooked if Apple wants to. As a consumer, I'm leaning AMD for my next build (and that's BEFORE Intel's self-inflicted wounds at Computex).
Intel's architecture, strategic platform differentiators (from Thunderbolt to Optane), predilection and preferential treatment to Apple, and platform efficiency cumulatively matter more to Apple than any advantage AMD could tempt them with, perf-per-dollar included. Trust me, Apple will
never use a CPU for Mac designed by AMD. Never ever ever. They're as likely to use a CPU for mobile designed by AMD.
2) Hardware is going nowhere but more power-efficient as a function of time (both Intel and AMD), so thermal solutions that work well today will continue to work regardless of the hardware it's cooling.
Intel has had a persistent growing advantage relative to AMD in this respect. By the time AMD matches Intel's efficiency today, Intel will be years ahead. That's not tenable for Apple given their industrial design and product feature priorities. Heck, Apple is frustrated with these things with
Intel already, since they know that they're actually better at this stuff on ARM than Intel is on x86.
3) If Apple ditches x86, they'd need to support legacy x86 software (somehow) or risk losing market share. I'd guess they would go carefully and probably see how Microsoft fares with Windows on ARM. That being said, Apple is reportedly doing it's own GPU design for an upcoming mobile CPU, so maybe they'll decide to vertically integrate to the fullest extent (OS + hardware).
Apple has the most experience of any company doing this - they created
Rosetta when migrating Macs from PowerPC to Intel/x86 to ease that transition, and it did the job. Of course, it would help a lot
if they proliferated a popular programming language years ahead of time, that can be compiled both to x86 and ARM...